This invention relates in general to power line communications. More specifically, it provides various improvements for systems wherein a central station "master" monitors and communicates with a plurality of "slave" units using a power line as a transmission line for communications.
A security system is one example of a system that requires centralized monitoring of slave units by a master. Various sensors such as infrared intrusion, window - glass damage and fire detection sensors are installed in target areas to be protected. These sensors are connected to a supervisory unit through individual transmitters and circuits respectively for centralized monitoring.
A problem of conventional "wired" master/slave systems is that, as the number of slave units and monitoring range increase, the amount of wiring required becomes excessive. A power line providing power to the various slave units can be used for communications to reduce the amount of wiring, but there are many factors which make it difficult to communicate reliably over a power line. For example, it is usually necessary to provide an arrangement for the prevention of signal line disconnections and quick detection of such disconnection faults.
Various schemes have been proposed to establish and maintain communications over a commercial power line. A transmission line generally utilizes single side band modulation for data signals, whereas a frequency or phase modulation is used for a distribution line. However, a power line is not designed for signal transmission.
It is electrically noisy, has a wide range of impedances, and its transmission characteristics fluctuate with line load. As a consequence, reliable signal transmission and particularly high speed data transmission have not been possible using conventional techniques.
There has been study undertaken in the applications of so called "spread spectrum" communications. The Journals of the Institute of Electronic and Communications Engineers of Japan, Sept/82, p 965 & Oct/82, p 1063, for example, disclose the principles of and comments on the applications of spread spectrum technology.
A spred spectrum communications system relies on so-called Pseudo-Noise (PN) diffusion or direct diffusion. Thus, a narrow-band data signal is transmitted over a wide-band transmission line by diffusing the spectrum thereof using an M sequential code as spurious noise signal, and even if the transmission characteristics of the transmission medium have a plurality of zero points resulting from the line load, a transmitted signal will not be substantially affected thereby. Moreover, even if narrow-band noise is blended with a transmitted data signal, the S/N ratio can be improved using correlation at the receiver.
However, the application of spread spectrum technology to power line communication systems permitting one master unit to simultaneously monitor a plurality of slave units still poses problems. For example, if multiple slave units simultaneously send data signals to the master unit, the data signals overlap and cannot be discriminated from one another. To prevent the slave units from sending the data signals to the master unit simultaneously, polling schemes have been used. In effect, the master takes turns looking at each slave successively to see if a given slave has a message to send to the master. Such systems require additional hardware, however, such as a CPU to control the polling, and such hardware is expensive.